1. Field of the Invention
The present invention relates to an optical encoder and a method of adjusting its output signal level.
2. Description of the Related Art
Encoders are used for detecting displacement in the linear direction in machine tool stages and three-dimensional measuring instruments. Optical and magnetic encoders are also used for detecting a rotational angle of servo motors and the like.
An optical encoder is generally composed of a scale fixed to a member for detecting displacement of a stage or the like, and a sensor head for detecting displacement of the scale. The sensor head includes a light source for emitting light to the scale, and a light detector for detecting a light beam transmitted, reflected or diffracted from the scale, and the movement of the scale is detected by change of the received light signal.
As a prior art, a representative optical encoder is explained by referring to FIG. 23. FIG. 23 is a block diagram showing a laser encoder of a prior art using a surface-emitting laser and a reflection type scale.
Such a laser encoder using a surface-emitting laser and a reflection type scale is disclosed, for example, in Jpn. Pat. Appln. KOKAI Publication No. 2002-48602.
This encoder is composed of a reflection type scale 20 and a sensor head 30 as shown in FIG. 23. An optical pattern 23 for detecting a moving distance is formed on a surface of the scale 20, and this pattern is made by patterning a member of high reflectivity made of an aluminum or the like on a surface of a transparent member of glass or the like. The sensor head 30 has a light detector 37 for detecting the moving distance formed on a semiconductor substrate 34, and a coherent light source (hereinafter called light source) 321 for detecting the moving distance disposed on the semiconductor substrate 34. The relative positional relation of the light source 321 and the light detector 37 is kept constant.
The scale 20 cooperates with a stage (not shown), and moves in the arrow direction in FIG. 23 relatively to the sensor head 30, and the sensor head 30 detects its moving distance by the change of intensity of a diffracted light from the scale 20. The detection signal of the moving distance is produced as a waveform as shown, for example, in FIG. 24. Herein, phase A and phase B are waveforms produced along with the movement of the scale 20, are generally quasi sinusoidal waves. Phase A and phase B are outputs different in phase by 90 degrees, and from the relation in phase of signals of phase A and phase B, the moving direction of the scale 20 can be detected. The scale 20 changes its position while maintaining a positional relation capable of forming a so-called Talbot image relatively to the sensor head 30.
The Talbot image is explained by referring to FIG. 25. For the sake of simplicity of explanation, a transmission type encoder is assumed, and it is discussed same also in a reflection type encoder.
As shown in FIG. 25, parameters are defined as follows:                z1 is a distance between a light source 1 and a surface of a scale 2 having diffraction grating formed thereon;        z2 is a distance between the surface on the scale 2 having the diffraction grating formed thereon and a light detector 3;        p1 is a pitch of the diffraction grating on the scale 2; and        p2 is a pitch of a diffraction interference pattern on a receiving surface of the light detector 3.        
The “pitch of the diffraction grating on the scale 2” is a spatial period of an optical pattern modulated in optical characteristic and formed on the scale 2.
The “pitch of a diffraction interference pattern on a receiving surface of the light detector 3” is a spatial period of an intensity distribution (light intensity pattern) of the diffraction pattern formed on the receiving surface.
According to the diffraction theory of light, when the z1 and z2 defined above are in a specific relation satisfying the relation represented by the following formula (1), a light intensity pattern similar to the diffraction grating pattern of the scale 2, or so-called Talbot image, is formed on the receiving surface of the light detector 3:(1/z1)+(1/z2)=λ/(k(p1 )2)  (1)where λ is a wavelength of a light beam emitted from the light source 1; and k is a natural number.
At this time, the pitch p2 of the diffraction interference pattern on the receiving surface can be expressed by the following formula (2):P2=p1×(z1+z2)/z1  (2)
When the scale 2 displaces in the pitch direction of the diffraction grating relatively to the light source 1, the light intensity pattern of the diffraction interference pattern is moved in the dislocating direction of the scale 2 while keeping the same spatial period.
Therefore, when a spatial period p20 of the photo detector 4 of the light detector 3 is set in the same value as p2, every time the scale 2 moves in the pitch direction by distance of p1, a periodic intensity signal is obtained from the light detector 3, so that the displacement of the scale 2 in the pitch direction can be detected.
Back to FIG. 23, the light source 321 for detecting the moving distance, the optical pattern 23 for detecting the moving distance, and the photo detector of the light detector 37 are disposed in a positional relation capable of forming the Talbot image, and a light and dark pattern similar to the optical pattern 23 for detecting the moving distance formed on the scale 20 is projected on the photo detector of the light detector 37. The period of this light and dark pattern is the period p2 calculated by the formula (2). The photo detector on the light detector 37 is formed to have this period of p2, and with this photo detector, the movement of the light and dark pattern can be detected.
Since the optical encoder is high precision, high resolution, and contact-free type, and is excellent in electromagnetic wave interference tolerance, the optical encoder is used in wide fields, and in particular the optical type is in the mainstream in encoders demanding high precision and high resolution.